They could be automatically checking out-of-state plates as they drive by, check the owner against the FL driver license database, and see if there is a match. If there is, ticket goes out in the mail. You may or may not be guilty of anything, but you'll have to show up in court to contest it, or get an attorney to do it for you ($$$). Moral of the story: be sure to get your DL from the state you obtain your license plates. Nowadays, automatic license plate readers with automatic registration checks are commonplace. If the out-of-state license plate is expired, they could mail a citation there too, but it probably won't mean a thing unless you're stopped much later on in Florida. It won't mean anything in your home state since its not a moving violation. Like msmith1199 said above, I believe the whole thing is a scare tactic. If I got one, it'd go right into the trash can. Same place those red-light camera notices go.
A number of years ago, Michigan cracked down on Michigan residents obtaining Ohio license plates, since Ohio is much cheaper than Michigan. The Michigan resident would live near the state line, and their mailing address would be Toledo, even though they lived in Bedford Township, Michigan. What Michigan authorities did was go around the neighborhoods near the state line at night and observe which cars had Ohio tags, ran them to make sure they came back to the Michigan resident they were parked near, and cite the owner (and in some cases, tow it). Didn't take too many of those to get the rest to fall in line. That's nearly impossible to do now since the usage of automatic geolocation by Ohio BMV, which was brought on to make sure each city, village, and township got the appropriate tax they were due from the registrations.
